526
Inorg. Chem. 1987, 26, 526-530
in isolating the unstable cis-[PtC12(HL1),] is clearly the result of the unusual synthetic approaches provided by the ready protonation-deprotonation equilibrium associated with the presence of the fluorinated alcohol function in the molecule. Conclusions. This study has significantly extended the range of known platinum(I1) complexes of P-0 hybrid ligands to include a phosphino alcohol in both monodentate and bidentate modes of coordination. W e have shown the value of a soft phosphine ligand in stabilizing the Pt-0 bond, and we have again demonstrated that steric, electronic, and kinetic factors may all be significant in determining the geometry of a platinum(I1) complex formed in any particular situation.
Acknowledgment. W e are grateful for financial support provided by the Natural Sciences and Engineering Research Council of Canada in the form of operating grants to N.C.P. and C.J.W. and a scholarship to C.D.M. Supplementary Material Available: For the structure determinations of compounds 2 and 5, tables of hydrogen atom positional parameters (Table S I ) , anisotropic thermal parameters (Tables S-I1 and S-HI), supplementary bond distances and angles (Tables S-VI and S-VII), and weighted least-squares planes (Tables S-IX and S-X), and for 2, a table of selected torsion angles (Table S-VIII) ( I O pages); tables of calculated and observed structure factors (Tables S-IV and S-V) (51 pages). Ordering information is given on any current masthead page.
Contribution from the Department of Chemistry, University of California, Santa Barbara, California 93 106
Nucleophilic Activation of Carbon Monoxide. 4. Dihydrogen Reduction of the Methoxycarbonyl Adduct R U ~ ( C O C02CH3))~~( Douglas J. Taube,’ Rndrzej Rokicki, Martin Anstock,2 and Peter C. Ford* Received August 5, 1986
The anionic triruthenium cluster R U ~ ( C O ) , ~ ( C O ~ C reacts H ~ )with - dihydrogen in dry THF to give methyl formate plus the hydride cluster H R U ~ ( C O ) ~Use ~ - .of D2 instead leads to the formation of DC02CH3plus DRu3(CO),,- as shown by 2H NMR. The rate of the hydrogenation is demonstrated to be first order in [H2] but inhibited by CO. These observations are interpreted in terms of a mechanism by which the principal pathway for reduction of the cluster involves the reversible dissociation of coordinated CO followed by rate-limiting H2 addition to the unsaturated intermediate. Details of the synthesis and spectroscopic properties of the Ru3(CO),,(CO2CH3)-anion are also described.
Introduction Investigations in this laboratory have been concerned with the activation of coordinated carbon monoxide by oxygen nucleophiles Such reactions with OHsuch as hydroxide or methoxide are key to Co activation in mechanisms for homogeneous of the water gas shift reaction in alkaline solution,+* while CH30activation is involved in certain methoxide cocatalyzed processes such as the carbonylation of methanol9 and the reductive carbony1ation Of nitroarenes”O Methoxycarbonyl adducts, as M~(C0)y-l(C02CH3)-~ are likely intermediates in the latter t a b i s cycles? Other adducts ‘1 are proposed to be
Taken in part from: Taube, D. J. Ph.D. Dissertation, University of California, Santa Barbara, 1985. NATO Postdoctoral Fellow, 1983. (a) Anstock, M.; Taube, D.; Gross, D. C.; Ford, P. C. J . Am. Cbem. SOC.1984, 106, 3693-3697. (b) Taube, D.; van Eldik, R.; Ford, P. C. Organometallics 1987, 6, 125. (a) Gross, D. C.; Ford, P. C. Inorg. Cbem. 1982, 21, 1702-1704. (b) Trautman, R. J.; Gross, D. C.; Ford, P. C. J . Am. Cbem. Soc. 1985, 107,2355-2362. (c) Gross, D. C.; Ford, P. C. J . Am. Cbem. SOC.1985, 107, 585-593. Ford, P. C. Acc. Cbem. Res. 1981, 14, 31-37. Halpern, J. Comments Inorg. Chem. 1981, 1 , 1-15. Laine, R. Aspects of Homogeneous Catalysis; Ugo, R., Ed.; D. Reidel: London, 1984; Vol. 5 , pp 217-240. Ungermann, C.; Landis, V.; Moya, S. A,; Cohen, H.; Walker, H.; Pearson, R. G.; Rinker, R. G.; Ford, P. C. J. Am. Cbem. SOC.1979, 101, 5922-5929. (a) Chen, M. J.; Feder, H. M.; Rathke, J. W. J . Am. Cbem. SOC.1982, 104, 7346-7347. (b) Braca, G.: Sbrana, G.: Barberini. C. C, Mol. Chem. 1984, 1 , 9-20. Alper, H.; Hashem, K. E. J . Am. Cbem. SOC.1981, 103, 6514-6515
0020- 166918711326-0526$01.50/0
the labilized species in nucleophile-catalyzed CO exchange and other substitution reactions -of metal -carbonyl comp6xes.ll However, few such adducts have been subjected to direct kinetics investigation. In this context, we have undertaken a quantitative investigation of certain reactions of the triruthenium methoxycarbony~anion Ru3(CO),,(C02CH3)- (I), which can be prepared via the reaction of methoxide salts with the neutral cluster R u ~ ( C O ) , , . ~In ~~* previous reports3 from this laboratory, it was shown that I is dramatically activated toward ligand substitution reactions relative to R U ~ ( C O )and , ~ that this enhanced lability apparently also leads to a markedly greater reactivity toward dihydrogen:3
\JI
A similar reaction of the mononuclear (ethoxycarbony1)cobaIt carbonyl species C O ( C O ) ~ ( C O ~ C , H (to~ )give ethyl formate plus Co2(CO),) has also been reported and investigated mechanistically by Ungvgry and Mark6.I2 In that case the parent carbonyl would be the unstable cation C O ( C O ) ~ + . Described here is a kinetics investigation of the reaction of Ru,(CO), ,(CO,CH,)- with dihydrogen. Also summarized are the details of the synthesis procedure and spectroscopic properties of this anionic cluster, parts of which have been described earlier. ( 1 1) (a) Morris, D. E.; Basolo, F. J . Am. Cbem. SOC.1968, 90, 2531-2535. (b) Hui, K.-Y.; Shaw, B. L. J . Organomet. Cbem. 1977, 124, 262-264. (c) Brown, T. L.; Bellus, P. A. Inorg. Cbem. 1978, 17, 3726-3727. (d) Basolo, F. Inorg. Chim. Acta 1981, 50, 65-70. (e) Darensbourg, D. J.; Gray, R. L.; Pala, M. Organometallics 1984, 3, 1928-1930. (f) Lavigne, G.; Kaesz, H. D. J . Am. Chem. SOC.1984, 106, 4647-4648. ( 1 2) Ungvlry, F.; MarkB, L. Organomerallics 1983, 2, 1608-1 61 2.
0 1987 American Chemical Society
Nucleophilic Activation of Carbon Monoxide
Experimental Section All manipulations were conducted under dry dinitrogen or CO using Schlenk techniques. Solvents were dried by using standard methcds13and distilled under N2. The parent cluster R u ~ ( C Owas ) ~ ~prepared by literature proced~res.'~Methanolic solutions of NaOCH, were prepared by the addition of small amounts of sodium metal to freshly distilled methanol. The solutions were freeze-pump-thaw degassed and stored under N2. Tetrabutylammonium methoxide was prepared from [NBu4]0H( 1 .O M in methanol, Aldrich) as de~cribed.~ The methoxide solutions were standardized by titration against oxalic acid using phenolphthalein as the indicator. Bis(triphenylphosphorany1idene)ammonium chloride ([PPNICI) was placed in vacuo at 100 OC for 3 h to remove traces of water. The PPN' salt of I was prepared from Ru,(CO),, as follows. Equimolar amounts of Na[OCH3] (2.0 M in CH,OH, 0.15 mmol) and dry [PPNICI, dissolved in the minimum volume of anhydrous methanol (about 2 mL), were successively added to a stirring solution of Ru,(CO),~ (50 mg, 0.078 mmol) in CO-saturated tetrahydrofuran (THF, 12.5 mL). The bright red solution was then syringed dropwise (over a period of 30 min) to stirred, CO-saturated hexanes (about 100 mL) cooled to -78 OC. The resulting orange solid was collected by filtration under CO. This product was subsequently dissolved in a minimum amount of CO-saturated THF, passed through a fine glass frit to remove the insoluble NaCl impurity, and then recrystallized by slow addition of the filtrate solution to hexanes at -78 OC. The yields for this reaction approached 90%. Anal. Calcd for [PPN][ R U ~ ( C O ) ~ ~ ( C O ~(C43H39013NP2Ru3): CH~)] C, 48.67;H,2.73;N,t 16;P,5.13. Found: C , 4 8 . 5 6 ; H , 2 . 7 9 ; N , l . t 8 ; P , 5.23. Electronic spectra were recorded on a Cary 118 spectrophotometer, infrared spectra on Perkin-Elmer 683 and Digilab FTS 60 spectrometers, and IH and I3C NMR spectra on a Nicolet 300-MHz NMR spectrometer operating in the pulsed FT mode with deuterium lock. The 2H NMR spectra were recorded in THF without lock. Some NMR spectra were obtained by using the 500-MHz instrument of the NSF Regional NMR Facility at the California Institute of Technology. Kinetics studies were carried out using R U , ( C O ) ~ ~ ( C O ~ CsoluH~)tions prepared in situ by the addition of concentrated [NBu,] [OCH3]or Na[OCH,] solutions in methanol to THF solutions of Ru,(CO),, (1.3 M). A stock solution of R U , ( C O ) (~5~mL) under H2 was syrX inged into UV/vis cells with a gas reservoir filled with the appropriate gas and the solution was thermally equilibrated (10 min). The dihydrogen partial pressures for different runs were varied by the use of manometer techniques. A Schlenk-fitted quartz cuvette was used for total pressures less than 1 .O atm, but a thick-walled glass cell was used for pressures greater than 1.0 atm. The reaction was then initiated by adding the methoxide solution (25 kL, 0.1 12 M) by microliter syringe, and the resulting optical density changes were followed at 460 nm by using a Cary 118 spectrophotometer fitted with a thermostated cell compartment. The dependence of the reaction rates on the partial pressure of CO was determined by using a similar technique. In a quartz cuvette attached to a glass reservoir of 139.3 mL total volume, a THF solution of Ru,(CO),~was equilibrated with a gas mixture consisting of 1.0atm of H2 to which various volumes of CO (0.33-20 mL) were added via a gastight syringe. With this technique, partial pressures of CO over the range 0.0025-0 15 atm were achieved. Results and Discussion Properties of Ru3(CO) (C02CH3)-. The electronic spectrum of this anion in methanol solution has previously been d e ~ c r i b e d . ~ In the presence of excess Na[OCH3], I displays two absorption bands, a maximum at 460 nm (e = 4.0 X lo3 M-I cm-I) and a shoulder at 370 nm ( 6 = 5.8 X lo3 M-I cm-l ). In 90/10 T H F / C H 3 0 H solution, only a small excess of methoxide is required to give an equivalent spectrum, in agreement with the larger equilibrium constant for formation of Ru3(CO),I(C02CH3)-in the less polar ~ o l v e n t . ~ Solutions of the [PPN] [ R U ~ ( C O ) ~ ~ ( C O ~salt C H in ~ ) CO] saturated THF displayed infrared bands at 2014 S (sometimes split into two equally intense bands at 2017 and 2010 cm-I), 1995 s, 1962 m, and 1637 m cm-'. The first three bands can be attributed to terminal carbonyls but occur at frequencies lower than ~ ~ to the negative those for the terminal CO's of R U , ( C O ) owing (13) Taube, D. J.; Ford, P. C. Organometallics 1986, 5, 99-104. (14) Eady, C. R.;Jackson, P. F.: Johnson, B. F. G.; Lewis, J.; Malatesta, M. C.; McPartin, M.; Nelson, W. J. H. J . Chem. Soc., Dalton Trans. 1980, 383-392.
Inorganic Chemistry, Vol. 26, No. 4, 1987 521 charge on I. The fourth band is attributed to the C02CH3-group. Notably, the N a + salt of I prepared in situ by the addition of Na[OCH3] to R U ~ ( C O )(about , ~ 0.005 M) in CO-saturated T H F shows a slightly different spectrum with uco's at 2017 s, 1995 s, 1966 m, and 1596 m cm-I. The lower frequency of the band for the -COzCH3 functionality as well as the slightly higher frequencies of the terminal CO stretches can be attributed to intimate ion pairing between Na' and the C 0 2 C H 3group drawing charge from the cluster. (Such ion pairing was not seen under the dilute conditions used for the kinetics studies.) A similar phenomenon has been described for the mononuclear M(CO),C02CH3- anions ( M = Fe, Ru or OS)^ as well as other metal carbonyl anions.15 (It is notable that some of the frequencies listed above are slightly different than those reported from this laboratory p r e v i o ~ s l y . ~ ~ These differences can possibly be attributed to the use of a different IR spectrometer or, more likely, to the fact that the carbonyl frequencies of the methoxycarbonyl adduct are sensitive to small perturbations of the solvent medium.) When the [PNN] [ R U ~ ( C O ) , ~ ( C O ~ Csalt H ~was ) ] dissolved in T H F under 1.5 atm of I3CO (99% I3C), exchange of coordinated and free C O was rapid as would be expected from the proposed dissociative mechanism for the ligand substitution reactions of RU~(CO)~~(CO~CH Exchange ~ ) - . ~ at room temperature was complete within minutes as monitored by the IF. spectra. The formation of the methoxycarbonyl adduct is undoubtedly responsible for the methoxide catalysis of C O exchange between R U ~ ( C O )and , ~ free C O as described previousIy.l2" The spectrum of the I3CO-exchanged cluster showed the expected shifts of the vco bands to lower frequencies, displaying bands at 1960 s, 1951 s, 1920 m, and 1600 m cm-I. The IH N M R spectrum (300 MHz) of [PPN] [ R u ~ ( C O ) ~ ~ ( C 02CH3)]in CDzC1, displayed a singlet at 3.36 ppm and a multiplet at 7.5 ppm with relative integrated areas of 1:lO. These can be assigned to methoxycarbonyl methyl hydrogens (3) and the PPN+ phenyl hydrogens (30), respectively. The room-temperature I3C N M R spectrum of I3CO 99% en(3.2 X lo-* M) in THF-d8 riched [PPN][Ru3(CO),,(C02CH3)] recorded under a 50 kPa excess pressure of I3CO, displayed a very broad, weak resonance centered at aC = 185.3 and two sharp singlets at 207.9 and 188.9 ppm, the latter two in an approximate intensity ratio 22:l. At -95 "C, the low-temperature limit of the 300-MHz instrument, the spectrum consists of two sharp peaks 185.3 and 189.0 and a much of roughly equal intensity at ?iC stronger, broad resonance at 208.1 ppm. These spectral properties are in qualitative agreement with those reported for the Ru3(CO),,(C02CH3)-ion prepared and studied in situ in 2/1 T H F / C H 3 0 H (v/v) by Darensbourg et a1.12e These spectra can be interpreted12ein terms of the absorption at 185.3 ppm being the carbon of free CO, the peak at 189 ppm being the carbon of the methoxycarbonyl group, and the much more intense peak at 208.1 ppm being the remaining carbonyls of the cluster. This implies that site exchange among the 11 terminal C O S remains rapid even at -95 O C . The same result was obtained when the I3C spectrum was recorded on the Caltech 500-MHz (125.7 M H z for I3C) instrument at -105 OC. For comparison, it is notable that the I3C N M R spectrum of R U ~ ( C O )in, ~T H F is also a singlet at -100 OC, implying similar low-temperature fluxionality,16while the I3C N M R spectrum of the hydride ion HRu3(CO),,-in methanol-d, gives a singlet at room temperature but a structured spectrum at -85 0C.8317 The increased intensity for the resonance of the dissolved free carbon monoxide at the lower temperature is probably the result of a decreased rate of exchange between free CO and CO coordinated to this very labile cluster. (Free CO is observable as a sharp signal at 185.0 ppm at room temperature (15) (a) Collman, J. P.; Cawse, J. N.; Brauman, J . I. J. Am. Chem. SOC. 1972, 94, 5905. (b) Sakakura, T.; Kobayashi, T.; Tanaka, M. C, Mol. Chem. 1985, I , 219-230. (16) (a) Aime, S.; Gambino, 0.; Milone, L.; Sappa, E.; Rosenberg, E. Inorg. Chim. Acta 1975, 15, 53-56. (b) These observations were reconfirmed in these laboratories by using a 300-MHz instrument. (17) Johnson, B. F. G.; Lewis, J.: Raithby, P. R.; Suss-Fink, G . J . Chem. Soc., Dalton Trans. 1979, 1356-1361.
528 Inorganic Chemistry, Vol. 26, No. 4, 1987
08-
\\m
Taube et al.
I
A
HRu.(COI.;
+ HCO-CH.
V
I
-
.E 2 0 -
07-
m I
06-
5
0504-
03-
0
02-
I
i .o
0.5
2.0
1.5
PH2 (in o t m ) O 300 ' t
350
400
450 )I
500
550
600
(I""
-
Figure 2. Plot of k d vs. P Hfor ~ the reactions of methoxycarbonyl cluster anions with H2 in THF. RU~(CO)~~(CO~CH,)+ H2 HRu3(CO),,+ HCO2CH3.
Figure 1. Temporal absorbance changes resulting from the reaction of
H2with Ru3(CO),,(C02CH3)-inTHF. Spectrum A is that of Ru3(CO),* in THF under H2. Spectrum B is that of the methoxycarbonyl adduct immediately after adding [NBu4][CH30].Spectrum C is that of HRU~(CO)~,after 2-h reaction. under analogous conditions in the absence of the cluster.) Reaction with Dihydrogen. THF solutions of Ru3(CO),, (1.1 X M ) under CO display a strong absorption band in the electronic spectrum centered at 390 nm (e = 7.9 X lo3M-' cm-' ) and a minimum at 350 nm (e = 4.0 X lo3 M-I cm-' ). The identical spectrum was observed under a H2/Co atmosphere, and no spectral changes occurred over an extended period. However, when a 2-fold excess of [NBu,] [OCH3] was added to T H F solutions of R U , ( C O ) , ~ the , bright yellow solution immediately turned red and new absorption bands appeared at 460 nm (e = 4.0 X lo3 M-' cm-' ) and 360 nm ( 6 = 5.8 X lo3 M-' cm-I )> indicating formation of Ru3(CO)],(C02CH3)-. Under C O the spectrum did not change further, but under Hz or H J C O atmospheres, there was a slow decrease in the 460-nmband and the growth of a new one at 386 nm (Figure 1). Throughout the reaction with H,, isosbestic points at 575,413,and 369 nm were maintained; thus no intermediates of detectable concentrations were observable. The product spectrum in Figure 1 displayed ,,A at 387 nm, consistent with the formation of HRu3(CO), in >95% yield according to expected optical density changes.l3 This product was also confirmed by infrared and IH N M R spectral changes. A T H F solution of [NBu,] [Ru3(CO)ll(C02CH3)] (prepared in situ ) ~ ~0.004 M [NBu,] [OCH,]) displayed from 0.002M R U ~ ( C Oplus uco bands at 2010 s, 1990 s, 1962 m, 1637 w, and 1596 w cm-'. When H2 was bubbled through this solution, these disappeared and new bands at 2064 w, 2006 s, 1983 s, and 1962 m cm-I characteristic of [HRu,(CO), appeared. A strong band at 1712 cm-I, assigned to the carbonyl stretch of methyl formate, also appeared. (The vco expected at 1725 cm-I for the bridging carbonyl of HRu3(CO), was apparently obscured by the much stronger band due to methyl formate under these conditions.) For the N M R experiment, the reaction of [NBu4][Ru3(CO)II(C0,CH3)](0.013M ) with H2 was carried out in THF-d8 solution. After the mixture was bubbled with H2new resonances appeared at 6 -12.67,3.74and 8.05 ppm, which can be assigned to the hydride of HRu,(CO), I - plus the methyl and formyl hydrogens of H C 0 2 C H 3 ,respectively. Traces of methanol as evidenced by the methyl hydrogens at 6 3.40were also observed. The HH~ F) -gave analogous reaction of Dzplus R U ~ ( C O ) ~ ~ ( C O ~inC T resonances at -12.5 ppm (DRu3(CO),,-) and 8.02 ppm (DCO,CH,) in the ,H N M R spectrum. Stoichiometries under conditions where the H, was introduced by bubbling through the solution (as determined from the integrated IH intensities) gave less than a one-to-one methyl formate to ruthenium hydride ratio; however, this discrepancy can be attributed to the high volatility of the methyl formate (bp 31.5 "C). When the reaction of [PPN] [Ru3(CO),I(C02CH3)] with H, was instead investigated in situ in a closed system (in this case, using CD2C12as solvent and a 5-mmN M R tube fitted with a Young valve), the resonances
10
-I
6- 9 -
0 2
3
876 54-
9
3-
-s
-
20
100"
r
200
Pc(;'
r
'
"
ot m
-'
c
300 ' "
l
'
I
400
Figure 3. Plot of /cobdvs. (Pco)-l for reaction of Ru3(CO),,(C02CH,)' with H2 (1.0 atm) in THF (25 "C).
at -12.67,3.74,and 8.05ppm appeared with the integrated ratios 1.0:3,0:~ 1, the integration of the formyl hydrogen being somewhat perturbed by the close resonances of the PPN' hydrogens. Thus, the stoichiometry of eq 3 is confirmed. However, in each case, a small amount of methanol was detected in the product mixtures as evidenced by a resonance at 3.40ppm. A probable explanation for this is hydrolysis of the methoxycarbonyl adduct by traces of water in the system. A brief IH N M R investigation of the reaction of [PPNJ[Ru3(CO),l(C0,CH,)] in THF-d, with H z was carried out over the range -80 to +10 "C. Over this temperature range, the reaction was quite slow and no intermediates or other interactions between the substrates were detected. Above 10 "C, reaction to form products was relatively rapid on the time scale of the N M R data collection and again no intermediates were observed. Kinetics Studies. The reactions of I with excess H2 were followed on a Cary 118 spectrophotometer. Plots of In ( A - A,) are linear, indicating a first-order dependence on Ru,(CO), ,(C0,CH3)- concentration. A plot of the resulting kobsdvalues vs. PH2is linear with a slope of 0.014s-l at& over the range 0.17-1.7 atm (Figure 2). Thus, one may conclude that the rate of this reaction is first order with respect to P H 2 .The reaction of I with D2 proved to be slower than that with H2, an observation which suggests that the dihydrogen in some form is directly involved in the rate-limiting step. When the reaction was carried out under a dihydrogen pressure of 0.79 atm, the measured kobsdvalue was 0.0098 s-' for H2 and 0.0071 s-I for D,. The ratio gives a kinetic isotope effect kH/kD of 1.4. Kinetics runs carried out under a pure H 2 atmosphere showed some long term deviations in the reaction end points, presumably owing to further reaction of H R U , ( C O ) ~ ~with - Hz to give H3R U ~ ( C O ) ~ ~ However, - . * ~ ~ ~ reactions carried out under HJCO mixtures gave stable end points consistent with the formation of H R u ~ ( C O ) ~as~the - only ruthenium product. Given that the reaction solutions of Ru3(CO)11(C0,CH3)- prepared in situ (18) (a) Bricker, J. C.; Nagel, C. C.; Shore, S. G. J . A m . Chem. SOC.1982, 104, 1444-1445. (b) Bricker, J. C.; Nagel, C. C.; Bhattacharyya, A . A.; Shore, S. G. J . Am. Chem. Sac. 1985, 107, 377-384.
Inorganic Chemistry, Vol. 26, No. 4, 1987 529
Nucleophilic Activation of Carbon Monoxide contained excess [NBu4][OCH)] or Na[OCH3], the effect of the methoxide concentration was examined briefly, but no rate difM.I9 ferences were noted over the 3-fold range, (0.4-1.2) X The Pco dependence of the reaction rate was determined for sets of runs using a fixed PH1( L O atm) but C O partial pressures ranging from 0.0025 to 0.15 atm. The reaction is dramatically suppressed by CO, and the plot of kobsdvs. Pco-I was linear with a zero intercept (within experimental uncertainty) (Figure 3). Thus, the kinetics for the reaction of I with H z follow the rate equation
Equation 4 would be consistent with the following mechanistic scheme:
'&
R U ~ ( C O ) ~ ~ ( C O ~ C H ~R)U- ~ ( C O ) ~ ~ ( C O ~ + CH CO ~ ) -( 5 ) k-s
R U ~ ( C O ) ~ O ( C O ~ C+ H ~H,) -
k6 +
H R u ~ ( C O ) I ~+- HC02CH3 (6)
This is a mechanism involving reversible dissociation of C O from I to give an unsaturated (or solvento) intermediate A followed by a rate-limiting reaction with HZ. Such a mechanism would display the rate law (7) For the condition where k-,[CO]
> k,[H,], this simplifies to
consistent with the behavior described by eq 4. On the other hand, if k6[H,] >> k-5[CO], then kobsdshould simplify to k5[I]; i.e., the rate should be independent of PH2.Such limiting behavior was not observed even in the absence of added CO; thus, the magnitude of k-5 may be such that C O labilized in eq 5 is sufficient to prevent this type of limiting behavior. While there have been no direct measurements of the reactivities of unsaturated cluster anions such as A with C O or similar ligands, the reaction of C O with R U ~ ( C O produced )~~ by flash photolysis in T H F has an estimated limiting rate constant of > lo3 S-I.~O The first-order limiting rate behavior apparently reflects the dissociation of T H F from Ru3(CO), I(THF); the methoxide adduct of this species should be much more labile. According to the above scheme, the ultimately limiting rate would be that of C O dissociation from I. Previous investigations of the reactions of I with P(OCH3)3(eq 2) concluded that k5 has a value of 5.8 s-l (25 "C) in 90/10 T H F / C H 3 0 H 3and 4.0 f 0.2 s-l ( 2 5 "C) in T H F . ] In contrast, the first-order rate constant kobsd for H R U ~ ( C O ) ,formation ~(eq 3) under the highest P,, investigated (1.7 atm) was 2.3 X s-l, several orders of magnitude below the above limit. If eq 8 were valid, the k&sd values plotted in Figure 3 would equal k5k6[H2]/k-5[CO].The slope of the plot is ( 2 . 5 0.1) X atm s-I, which can be translated to (2.1 0.1) X lo-' L mol-] s-I by taking into account the estimated solubility of C O is T H F (8.4 X lo-, mol L-l atm-' at 25 0C).21 The concentration of H 2 under these conditions (PH,= 1.0 atm) would be 6.0 X given
*
~
~~
(19) It is notable that IR spectra of the RU,(CO)~~(CO,CH~)adduct at higher concentrations of Na[OCHJ in T H F displayed weak bands indicating the presence of low concentrations of another species, namely the proposed diadduct Ru3(CO),o(C02CH3),2-.k The independence of the hydrogenation rates to methoxide concentration indicates that this species did not have particular kinetics significance under the conditions investigated. (20) Desrosiers, M. F.; Wink, D. A.; Trautman, R.; Friedman, A. E.: Ford, P. C. J . A m . Chem. SOC.1986, 108, 1917-1927.
an estimated solubility of Hz in THF;,' accordingly, the rate constant ratio k5k6/k-5would be 3.5 X S-I. Given the value of 4.0 s-' determined for k5 (above),) the ratio k6/k-5 = 8.8 X can be calculated. This ratio represents the competitive reactivities of H2 (k6) and CO ( k 5for ) the proported unsaturated intermediate R U ~ ( C O ) , ~ ( C O ~ Cand H ~ )indicates that A is more than 5 orders of magnitude more reactive toward C O to re-form I than it is toward H, to give HRu,(CO),,-. For comparison, the tricoordinate 14-electron intermediate RhC1(PPh3), in benzene has been shown to be only 700 times as reactive toward C O than toward H,; the less reactive tetracoordinate species RhC1(PPh3), displays a reactivity ratio of about 4 X 104.22 The failure to detect measurable concentrations of intermediates along the reaction coordinate of eq 6 restricts interpretation of the detailed mechanism of this complicated transformation. However, a logical sequence would be for dihydrogen to add to the dissociated intermediate Ru3(CO)10(C02CH3)-or its solvento analogue to give a dihydride species H2Ru3(CO)lo(C02CH3)(C), perhaps via the equilibrium precoordination of dihydrogen to give (H2)Ru3(CO)lo(C02CH3)-(B) followed by H-H bond cleavage (e.g., eq 9). The dihydride C may then undergo rapid reductive
+
R U ~ ( C O ) , ~ ( C O ~ C H , ) H2 -
km
(H2)Ru3(C0)10(C02CH3)-
H2Ru3(CO) IO(COZCH3)- (9)
elimination of H C 0 2 C H 3to give H R U ~ ( C O ) ~which ~ - , subsequently would scavenge C O to give H R u ~ ( C O ) ~ ~The - . kinetic isotope effect k H / k D= 1.4 is consistent with the values noted for various oxidative additions of dihydrogenZ3and indicates that the transition state of eq 6 probably involves partial breaking of the dihydrogen single bond. This plus the observation of the first-order dependence on PH2would indicate that the hypothetical dihydrogen species B could only be formed in small equilibrium concentrations under these conditions (i.e. K,[H,] 420 nm) irradiation of R U ~ ( C O ) ,yields substitution can be effected photochemically to give ~ r u n s - R u ( C 0 ) ~ ( C ~ H ~Near-UV ),. irradiation of Ru(CO),(C2H4) in rigid, , further C O loss to give C ~ S - R U ( C O ) ~ ( C ~ His, ) , C2H4-saturated, 3-methylpentane glasses at 90 K yields R u ( C O ) , ( C ~ H , ) ~but observed after only -5% consumption of RU(CO)~(C,H,). Isomerization of photogenerated C ~ ~ - R U ( C O ) ~ ( C ~toH trans-Ru,), (CO),(C,H,), is only observed on warming the glass above 210 K. Prolonged irradiation of photogenerated c ~ s - R u ( C O ) ~ ( C ~ H , ) ~ at 90 K yields loss of additional C O to give a monocarbonyl complex, formulated as Ru(CO)(CZH4),, which reacts on warming with photoreleased C O to initially regenerate cis-Ru(CO),(C,H,),. The photochemistry of Fe(C0),(C2H,) is the same as that of the R U ( C O ) ~ ( C ~ Hexcept ,) that ~ r u n s - F e ( C 0 ) ~ ( C ~could H ~ ) only ~ be detected by IR spectroscopy at temperatures below 210 K. The new results show that species previously formulated as Fez(C0)6(alkene)2 are in fact Fe(CO),(alker~e)~.In solution, M(C0)3(C2H4)2(M = Fe, Ru) and Ru(CO),(C,H,), are substitutionally labile and may serve as versatile reagents in preparative chemistry. Addition of deoxygenated 1-pentene to solutions of the bis- and tris(ethy1ene) complexes results in rapid catalytic isomerization at 293 K to a mixture of 2-pentenes, thus establishing the viability of both M(CO)3 and M ( C 0 ) 2 species as repeating units in the catalytic alkene isomerization. Deactivation of M(CO)3(alkene)2 as a 1-pentene isomerization catalyst, in the absence of excess CO, proceeds, at least in part, by dehydrogenation of 1-pentene to form the stable, catalytically inactive (at 298 K) M(CO),(q4- 1,3-pentadiene) complexes. Research in this group and elsewhere has established that an extraordinarily active alkene isomerization catalyst results from photolysis of Fe(C0)5 in the presence of alkenes.'-3 A carbonyl-bridged diiron complex4 and, alternatively, a mononuclear tricarbonyl iron ~ n i t lhave ~ , ~been proposed to carry the catalytic cycle. A report from this group5 establishes that iron carbonyl intermediates in the photocatalytic systems could be observed spectroscopically at subambient temperatures, including HFe(C0)3(q3-C3H5)from photolysis of Fe(C0)4(C&) in a rigid alkane glass at 77 K. In neat I-pentene, warmup of photogenerated HFe(C0)3(q3-C5H9)(from Fe(CO)5/ 1-pentene at 77 K) result in significant catalytic isomerization of 1-pentene above 243 K in the dark. Eventual regeneration of Fe(CO),(alkene) is accompanied by decline of catalytic activity. Fe(C0),(q3-allyl) radical species, also detected at 143 K in 1-3% yield as photoproducts of Fe(CO), and olefins, have been implicated in catalytic reactions of New findings reported here reveal the nature of the dominant species resulting from near-UV irradiation of Fe(CO)5/alkene solutions. Species previously formulated as Fe2(CO)6(alkene)25 are in fact mononuclear Fe(CO)3(alkene)2 complexes, consistent with a report by Fleckner, Grevels, and H e w 7 Other important mononuclear Fe species are reported herein including di- and monocarbonyl complexes. We have also extended the low-temperature photochemistry to Ru(CO),(alkene) systems and find *To whom correspondence should be addressed.
0020-1669/87/ 1326-0530$01 S O / O
that mononuclear bis- and tridethvlene) comdexes can be gen. . erated photochemically via sequential photodhemical reactions represented by eq 1-3 for the case of alkene = C2H4. Photo-
C2H4
< 400 nm) a,kane. 298 '
+ C2H4
hu ( A < 400 nm) alkane, 243 *
hv (A
Ru(C0)4(C2H4) +
Ru(C0)3(C2H4)2
+
t r u n ~ - R u ( C 0 ) ~ ( C ~ HC~O) ~(3) chemistry according to eq 1 is knowns-I0 and provides an excellent (1) (a) Mitchener, J. C.; Wrighton, M. S. J . Am. Chem. SOC.1981, 103, 975. (b) Schroeder, M. A.; Wrighton, M. S. Ibid. 1916, 98, 551; J . Orgunomet. Chem. 1971, 128, 345. (2) Chase, D. B.; Weigert, F. J. J . Am. Chem. SOC.1981, 103, 977. (3) Whetten, R. L.; Fu, K.-J.; Grant, E. R. J . Am. Chem. SOC.1982, 104, 4270. (4) Swartz, G. L.; Clark, R. J. Inorg. Chem. 1980, 19, 3191. Swartz, G. L. Ph.D. Dissertation, Florida State University, 1977. ( 5 ) Mitchener, J. C.; Wrighton, M. S. J . Am. Chem. SOC.1983, 105, 1065. (6) (a) Krusic, P. J.; Briere, R.; Rey, P. Orgunometallics 1985, 4, 801. (b) Krusic, P. J.; Filipo, J. S.,Jr.; Hutchinson, B.; Hance, R. L.; Daniels, L. M. J . Am. Chem. SOC.1981, 103, 2129. (7) Fleckner, H.; Grevels, F.-W.; Hess, D. J . Am. Chem. SOC.1984, 106, 2021.
0 1987 American Chemical Society
